Since 1984, processing power of microprocessor-driven, host personal computers have increased four-fold because of improvements in processor and disk technologies. Cached memory and full 32 bit data path capability, have contributed to the increase, as has the improvement in disk access times, from say 45 msec seek times to 17 msec using presently available disk drives.
I/O controllers and subsystems have also evolved to accommodate increased terminal loads and higher data rates. Examples of such I/O controller and I/O subsystems are described in the literature, and include:
(a) U.S. Pat. No. 4,162,520 for "Intelligent Input-Output Interface Control for Input-Output Subsystem" describing the peripheral-controller known as a line control processor which controlled and handled data transfers between a given peripheral terminal unit and the main host system;
(b) U.S. Pat. No. 4,074,352 for "Modular Block Unit for I/O Subsystem" describing a base submodule unit which housed and supported a group of eight peripheral-controllers and interfaced to a main host computer system;
(c) U.S. Pat. No. 4,106,092, for "Interface System Providing Interfaces to Central Processing Unit and Modular Processor-Controllers for I/O Subsystems" describing a unit in the main host system, designated as an I/O translator or "IOT", which controls and regulates data transfer between the main host system and a plurality of base submodules and their peripheral-controllers; and
(d) U.S. Pat. No. 4,189,769, for "Input/Output Subsystem for Digital Data Processor System", describing a subsystem wherein a plurality of peripheral-controllers are organized in base submodules for data communications with a main host system. The peripheral-controllers and the base submodule form an input-output subsystem for controlling data transfers to/from a large number of peripheral units to the main host computer system;
While the aforementioned I/O subsystems have enjoyed success, their complexed implementation sometimes have been carried out at the expense of large host overhead (normalized CPU time devoted to manage and coordinate operations) and poor throughput. Moreover many such subsystems have been in use in connection with conventional disk operating systems (DOS) and, as such, are not easily adapted for use in UNIX.RTM. operating environments where the host computer is a personal computer interfaced with a plurality of remote serial communicating devices.